This invention relates to solid propellants useful for rocket propulsion. More specifically, this invention concerns itself with a method for coating finely divided boron fuel particles with an ammonium perchlorate oxidizer and to the coated particles produced thereby which find particular application in composite solid propellants.
Propellant compositions find application as a means for imparting motion to an object, such as a rocket or guided missile. Two main classes of propellants are recognized on the basis of their physical characteristic and are referred to as either liquid or solid propellants. Solid propellants are further divided into two separate groups. The first group is referred to as homogeneous solid propellants while the second group, which the present invention concerns itself with, is referred to as composite solid propellants.
Conventional composite solid propellants generally are mixtures of a finely ground oxidizer and a binder of plastic, resinous or elastomeric material. The matrix material provides fuel for the combustion while the oxidizer, being a major constituent, contributes most to the burning characteristics of the propellant. The burning rate, stability to detonation, flame temperature, and other burning characteristics depend to a great extent on the particular oxidizer and its particle size. Formulations containing ammonium perchlorate, a well known oxidizer, have somewhat lower burning rates and flame temperatures, with little smoke production.
In an attempt to improve the burning characteristics of propellants employing an ammonium perchlorate oxidizer, it has been suggested that finely divided metal powders, such as Al, Mg or B, be added to the propellant mix as an additional fuel component. The metal particles are preferably about 0.25 to 50 microns in size. The amount of metal powder is not critical, but is determined by the specific use and characteristics of the propellant composite. Amounts of only one or two percent have provided some improvement. Generally, the metal constitutes a major proportion by weight of the propellant with maximum amounts being determined by the need to avoid granulation of the mixture and a deficiency in the amount of oxidizer. The use of the metal powders tends to increase the density and improve the specific inputs of the propellant because of the metal powders high heat of combustion.
However, with certain applications, such as inflight controllability of the burning rate of the propellant, the use of separate metal fuel and oxidizer components has proven to be undesirable. In an attempt to overcome the problem, it was found that boron particles coated with ammonium perchlorate provided improved burning characteristics for propellant compositions using these ingredients. The reduced total surface area of the coated boron particles, as compared with the use of separate particles of boron and ammonium perchlorate, reduced the total surface area of the solids incorporated into the propellant. This made possible increased solids loading and improved the processability of the propellant. Combining the metal and oxidizer ingredient also raised the pressure exponent of the propellant as compared with the use of separate components which improves the inflight controllability of the propellants burning rate.